An elementary gamma-ray scintillation spectrometer consists of the elements shown: a scintillating crystal to produce light flashes upon deposition of energy, a high voltage supply to drive the photomultiplier tube, a preamplifier to condition the signal for transmittal to the amplifier, the amplifier, a pulse height analysis device, and, finally, an output device.
Refer to the companion notes, "Gamma-Ray Scintillation Counting", for a discussion of components and the mechanisms that generate the detector output.
For a single channel device, the simplest system, the pulse height analyzer is a simple gate: the count is accepted if it is above the lower window setting and below the upper window setting. The system counts events which are within a defined energy range. Note that we can utilize this type of system as a spectrometer by taking successive readings of a (relatively long half-life) source with the window set to accept increasingly larger pulses, i.e., higher energy gammas. The data thus generated can be used to produce a spectrum of the gamma events as a function of energy.
These simple analyzers have few controls:
Let's look at these last two settings in the context of a the pulse height, or the quantity pulse height represents: energy deposited in the crystal:
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The analysis system has a fixed range for the pulses which it may evaluate. That range is indicated in the sketch. Changing the gain does not shift that range which the analyzer handles. However, increasing the gain of the amplifier which feeds the analyzer increases the pulse height, moving the position of the pulse height within the range. If for example, a pulse is located at A, increasing the gain will move it rightward, to B. Increasing the H.V. applied to the detector will produce the same result, and may be used on those devices that do not have a "GAIN" adjustment.
On the other hand, increasing the gain (or increasing H.V.) will bring pulses normally below the range of the detector (noise) into the range of the analyzer.
In summary, increasing the gain or H.V. decreases the range of energy pulses that may analyzed, while at the same time providing better definition over that smaller range, but at the risk of increasing unwanted noise.
A window within the range of the analyzer is established by setting a base energy level, usually expressed in percent of total range, and by setting an incremental aperture, also expressed in percent of total range. Setting the window as shown will result in pulse A being rejected (not counted), and B accepted. Note that in some systems the window is set by establishing the lower (base) level and an upper level rather than the window width.
The object of this experiment is to study the behavior of a simple, single-channel scintillation gamma detector.
Note the set-up of the scintillation detector system: components and wiring. Select a laboratory monoenergetic gamma emitter source and place it in the tray of the detector assembly. Turn the power on, then with the H.V. adjustment at minimum, turn on the H.V. Set the timer to a short value (0.5 to 1.0 minutes). Adjust the gain as follows in order to ensure that the range of gammas emitted from the sample fall into the largest usable range of the system:
Set the threshold at 70% and the window at 30%, or lower window to 70% and upper window to 100%, depending upon the analyzer. Take a count. increase the gain (or H.V.) and repeat for uniform increments of gain until the count takes a sizable increase. Take one more step. Plot counts/min vs. gain (or H.V.).
Note that the gamma events recorded for the monoenergetic source must be at or below the energy of the emitted gamma. With low gain (or H.V.) the pulses received by the analyzer will fall below the 70% threshold and be rejected (not counted). Only when the gain (or H.V.) is sufficiently high will the pulses pass the sort and be counted. Left at this setting, the photopeak will appear at roughly 70% of the range, establishing a reasonable operating span.
Set the timer for one minute counts. See that you get 1000 cpm or more.
Set the window for a window width of 10%, and take a series of 1 minute counts with the baseline set at increments of 10%, from 0 to 90%. Review your data and be sure that the photopeak exists at about 70-80% of the range surveyed.
Set the window width to 3% or lower and repeat the previous steps so as to cover the entire range of the analyzer.
